1. Field of the Invention
The present invention relates to a temperature and humidity controller for a constant temperature and humidity bath and, more particularly, to a temperature and humidity controller which is suitable for use with a constant temperature and humidity under high temperature and high humidity conditions.
2. Description of the Prior Art
Many kinds of constant temperature and humidity baths are now on the market as environmental test equipment for making temperature and humidity characteristics tests of electronic parts and various materials.
Conventionally, a psychrometer of the type including temperature sensors such as platinum resistance bulbs, thermocouples, or thermistors are employed as a humidity sensor for effecting humidity control in such a constant temperature and humidity bath, and in general, relative humidity is computed from the temperature difference between the dry and the wet bulb so that humidity control is effected accordingly. The humidity computing method by such a psychrometric method is known as defined under JIS (Japanese Industrial Standard) 8806.
Humidity sensors of other than the psychrometer are, for example, resistive and capacitive humidity sensors which utilize variations in their electrical characteristics which are caused by adsorption. They include an electrolytic lithium chloride humidity sensor, an organic high polymer film humidity sensor, a metallic oxide ceramics humidity sensor, etc.
A constant temperature and humidity bath control method employing such a humidity sensor is usually one that converts temperature signals of the dry and wet bulbs into a relative humidity signal and uses this signal to control the relative humidity in the bath.
Recent environmental test equipment is often required to withstand severe conditions of high temperature and high humidity. On this account, the conventional humidity sensors of the type utilizing the electrical characteristic variations by adsorption are scarcely used as the humidity sensor in the constant temperature and humidity bath, because they are not reliable in reproducibility, stability, responsibility, and interchangeability.
It is therefore a general practice in the art to employ the psychrometer as the humidity sensor in the constant temperature and humidity bath. However, the psychrometer necessitates the use of gauze and water in the wet bulb, and hence is most likely to make an error owing to strains on the gauze and fur on the wet bulb surface; accordingly, the psychrometer must be handled with the utmost care and its maintenance is very cumbersome. Besides, the conventional psychrometer has the shortcoming that an error tends to increase in a region of low temperature and low humidity.
Furthermore, it has recently been noted that the prior art method of converting temperature signals of the dry and wet bulbs into a relative humidity signal for controlling the relative humidity in the bath is not relaible, because both of the dry and wet bulb temperatures exert influence on the relative humidity control. As a solution to this problem, a method which effects control through the combined use of dew point and temperature or absolute humidity and temperature is now under study.